Makefile: Drop include/asm directory as well as symlink

At present when using 'make mrproper' on an out-of-tree build, a warning
is shown about include/asm being a directory. With old versions of U-Boot
it is a file, but more recently it has become a directory.

Remove this directory first, since that covers both cases.

Signed-off-by: Simon Glass <sjg@chromium.org>